1. Field of the Invention
This invention relates to a process for producing high-strength synthetic aggregates from calcium oxide (CaO) containing flue gas desulfurization (FGD) ashes, such as those generated from a fluidized-bed combustion (FBC) unit and the Limestone Injection Multistage Burner (LIMB) process; including a process for producing high-strength synthetic aggregates from calcium hydroxide (Ca(OH).sub.2) containing FGD ashes, such as those generated from duct sorbent injection and spray dryer processes.
2. Summary of Prior Art
There are several processes in the prior art for making pellets suitable as synthetic aggregates. These processes, such as shown in U.S. Pat. No. 4,377,414, utilize solid fly ash containing materials obtained from a lime based dry scrubbing flue gas desulfurization operation. In the U.S. Pat. No. 4,377,414, after the critical amount of water is added, the product is compacted under confined pressure at a critical compaction ratio of from about 2 to 3 so that the interstitial spaces between the fly ash particles are sufficient to accommodate the volumetric changes within the product without deleterious expansion during curing. In the present invention strong aggregates are produced on a rotary pelletization disc without the use of confined pressure.
A process is described in U.S. Pat. No. 4,490,178 in which granules are formed by mixing calcium oxide, fly ash, and water and curing said granules at temperatures between 30.degree. C. and 100.degree. C. Hydration of the calcium oxide to calcium hydroxide takes place during the granulation and curing steps. To control the maximum temperature of the granules during curing, heat absorbing particles are added with the granules. U.S. Pat. No. 4,880,582 also describes a curing system in which the pellets are cured in the presence of particulate fly ash. In the present invention, the hydration reaction occurs before the pelletization and curing steps.
U.S. Pat. No. 4,997,357 describes equipment to treat power station residues containing calcium oxide. The equipment includes mixers and rotary drums.
An apparatus for controlling the amount of water sprayed onto a tilted rotating cylindrical pelletization disc is described in U.S. Pat. No. 5,003,953. A moisture sensor detects the moisture in the pellets in the rotary disc. A signal from this sensor controls the amount of water sprayed onto the pelletization disc by means of a programmable controller. In the present invention, a sensor measures the amount on water contained in the wetted ash solids going to the rotary disc. Using this signal, the amount of water going to the mixer, before the rotary disc pelletizer, is controlled.